This invention relates to novel hot melt adhesive compositions based on polymer blends which consists of stereospecific, predominately crystalline syndiotactic polypropylene homo- and co-polymers (SPP) and amorphous atactic poly-xcex1-olefins (APAO). More particularly, this invention relates to adhesive compositions that find utility in case/carton sealing and in manufacturing nonwoven disposable articles such as diapers and feminine hygiene products. The adhesive compositions are particularly useful as an elastic attachment and construction adhesive in assembly of disposable nonwoven articles.
Hot melt adhesives typically exist as solid masses at ambient temperature and can be converted to a flowable liquid by the application of heat. These adhesives are particularly useful in manufacturing a variety of disposable goods where bonding of various substrates is often necessary. Specific applications include disposable diapers, hospital pads, feminine sanitary napkins, pantyshields, surgical drapes and adult incontinent briefs, collectively known as disposable nonwoven products. Other diversified applications have involved paper products, packaging materials, tapes and labels. In most of these applications, the hot melt adhesive is heated to its molten state and then applied to a substrate. A second substrate is then immediately brought into contact with and compressed against the first. The adhesive solidifies on cooling to form a strong bond. The major advantage of hot melt adhesives is the absence of a liquid carrier, as would be the case of water or solvent based adhesives, thereby eliminating the costly process associated with solvent removal.
For many applications, hot melt adhesives are often extruded directly onto a substrate in the form of a thin film by using piston or gear pump equipment. In this case, the substrate is brought into intimate contact with a hot die under pressure. The temperature of the die must be maintained well above the melting point of the adhesive, which is typically between 150 and 200xc2x0 C. For some applications, particularly for manufacturing nonwoven articles, bonding of delicate and heat sensitive substrates, such as thin gauge polyethylene films, is often involved. Direct contact between the film and the die, in these cases, must be avoided to prevent the film from burning or distorting. Several application methods have been developed through which a hot melt adhesive can be spray coated with the aid of compressed air onto a substrate from a distance. These non-contact coating techniques include spiral spray and various forms of melt-blown methods. Direct contact between the coating head and the substrate is thus eliminated. All the coating techniques herein described above are well know to those skilled in the art and commercial equipment is readily available.
The spray coating techniques, however, pose stringent requirements on hot melt adhesives. The viscosity of the adhesives must be sufficiently low, usually in the range of 2,000 to 30,000 cP, preferably in the range of 2,000 to 15,000 cP, at the application temperature. Many other physical factors, especially the rheological properties of the adhesive, come into play in determining the sprayability of a hot melt. The majority of commercial hot melt products do not lend themselves to spray applications. There are no accepted theoretical models or guidelines to predict sprayability and it must be determined empirically with application equipment.
In accordance with the present invention, it has been found that a mixture comprising a polymer blend of SPP and APAO, a tackifying resin, a plasticizer and, optionally, a synthetic polyolefin wax or petroleum wax provides a sprayable hot melt adhesive composition. The composition has novel combinations of properties including toughness, low or no shrinkage, high cohesive strength, low viscosity, excellent heat stability and good adhesion to a variety of substrates. The composition of the present invention is particularly useful in assembly of disposable nonwoven articles for bonding of polyethylene and polypropylene films, nonwoven fabrics and elastic strands to each other or to themselves.
One of the components in the polymer blend is the syndiotactic polypropylene (SPP) polymers. The SPP polymers useful in this invention are essentially high molecular weight stereospecific propylene homopolymers or copolymers of propylene with other xcex1-olefin monomers such as ethylene, butene-1 or hexene-1. The syndiotactic polymers should not be confused with the conventional crystalline polypropylenes and essentially amorphous atactic poly-xcex1-olefins (APAO). These polymers differ from each other in both structures and properties. It is well know to those skilled in the art that the conventional crystalline polypropylenes have an isotactic molecular chain configuration. The isotactic configuration can be described as having the methyl groups attached to the tertiary carbon atoms of successive monomeric units on the same side of a hypothetical plane drawn through the main polymer chain. This type of stereochemistry structure can be illustrated graphically by using the Fisher projection formula as the follow: 
Due to its high degree of chain regularity, the conventional isotactic polypropylenes (IPP) are highly crystalline with crystallinity typically greater than 50% and a heat of fusion greater than 70 J/g. They are usually stiff materials having high density and high melting point. Due to the lack of flexibility, an IPP polymer can only be used as a modifier in small amounts, typically around 2% to 5% by weight, in hot melt adhesive formulations. A typical conventional IPP usually has a melt flow rate, which is inversely related to the weight average molecular weight, in the range of 0.5 to 200 g/10 min as measured in accordance with ASTM D-1238 test method.
Another component of the polymer blend comprises an APAO polymer. APAO polymers are a family of essentially amorphous low molecular weight homopolymers of propylene or copolymers of propylene with ethylene or butene or hexene. In contrast to the regular structures in IPP or SPP, APAOs have atactic molecular chains with the methyl groups on the successive monomeric units sterically randomly distributed on both sides of the hypothetical plane through the polymer chain. The stereo configuration of the atactic APAO molecular chain can be illustrated graphically by using the following Fisher projection formula: 
The stereo chain structure of SPP is uniquely different from that of IPP and from that of APAO. In contrast to the isotactic chain configuration of IPP and the atactic chain configuration of APAO, the stereochemistry of SPP can be described as having the tertiary methyl groups of successive monomer units along the chain alternatively disposed on each side of the hypothetical plane. The stereo configuration of SPP can be depicted below: 
The stereo configuration of polypropylene can also be characterized quantitatively through C-13 NMR. In NMR nomenclature, a xe2x80x9cmesoxe2x80x9d dyad of successive methyl groups on the same side of the plane, as in the case of IPP, is represented by the letter m. A xe2x80x9cracemicxe2x80x9d dyad of successive methyl groups on the opposite sides of the plane, as in the case of SPP, is represented by the letter r. The percentage of m or r defines the degree of polymer tacticity with the sum of m and r equal to 100%. Thus, a perfect isotactic polypropylene will have 100% m dyad, whereas a perfect syndiotactic polypropylene will have 100% r dyad. This unique stereochemical structure of SPP results in an unusual and desirable combination of physical and mechanical properties such as low density, low melting point, flexibility and toughness.
The syndiotactic polymers used in the present invention preferably have an r value equal to or greater than 70%. The polymers having an r value greater than 80% are more preferred and those having an r value greater than 85% is most preferred. It should be pointed out that the r values of conventional IPPs, in comparison, are generally in a few percent range.
In addition to the difference in stereochemistry, SPPs are also readily distinguishable from IPPs and APAOs by their unique physical properties. Typical SPPs will have a melting point between 130 to 160xc2x0 C., whereas crystalline IPPs typically have a melting point about 176xc2x0 C. APAOs, on the other hand, are usually predominately amorphous without a well-defined melting point although some grades of commercial products may exhibit very low degree of crystallinity. Another profound difference between SPP and IPP and APAO lies in their densities. The density of SPP is typically between 0.86 to 0.90 g/cm3, which is in between those of IPP and APAO. IPPs have the highest density ranging from 0.90 to 0.95 g/cm3 and APAOs, the lowest ranging from 0.85 to 0.87 g/cm3.
Due to their high melting point, high degree of crystallinity and the lack of desirable physical and mechanical attributes such as flexibility and toughness, the conventional IPPs have not been used alone as the polymer base for hot melt adhesive applications. A hot melt adhesive based on IPP would be too brittle to offer acceptable bond strength and yet would require high application temperature that goes well beyond the melting point of the polymer.
Hot melt adhesives containing APAOs, APAO/polyethylene (PE) blends, APAO/polybutene (PB) blends, or APAO/IPP blends are known in the art. These adhesives typically consist of an APAO, or an APAO blend herein mentioned above, and a hydrocarbon type of tackifier. It is well know that adhesives based on APAOs generally have poor cohesive strength, poor heat resistance, low elevated temperature bond strength and low shear values. APAOs have not found much use in disposable nonwovens applications where a combination of high bond strength at very low coating weight and easy processibility by spray techniques is required. The APAO based adhesives usually lack such capabilities. Although various attempts were made to address these problems by blending APAO with PE, PB and the conventional IPP, very often such modifications not only failed to rectify the problems, but also led to adverse side effects.
For example, Trotter et al, in U.S. Pat. No. 4,022,728, describes a hot melt pressure sensitive composition comprising a mixture of APAOs, a low molecular weight substantially amorphous elastomer, a liquid tackifier and a conventional crystalline polypropylene (IPP) in the amount of up to 2% by weight. It is claimed that the composition provides good adhesive properties at low temperatures.
Meyer et al, in U.S. Pat. No. 4,120,916, discloses hot melt adhesive compositions comprising a blend of low molecular weight polyethylene, low molecular weight conventional propylene containing polymer and APAO. These adhesive compositions are said to offer short open time and to be useful for bonding of paraffin modified corrugated board.
Lakshmanan et al, in U.S. Pat. No. 4,761,450, discloses a polymer blend useful as hot melt adhesive comprising a low density ethylene polymer, a copolymer of butene-1 with ethylene or propylene, a hydrocarbon tackifier and a low molecular weight polymer consisting of a low molecular weight liquid polybutene, an amorphous polypropylene and mixtures thereof.
Lakshmanan et al, in U.S. Pat. No. 5,478,891, also discloses blend compositions containing (a) a high molecular weight copolymer of ethylene with an xcex1-olefin having at least 4 carbons and (b) an amorphous polypropylene or amorphous polyolefin. The components of the blends are described as having molecular weight range between 300 to 6000. The polymer blends are claimed to be useful for hot melt adhesives, coatings, sealants, asphalt modifiers and plastic additives.
Ryan discloses in U.S. Pat. No. 5,747,573 an APAO based hot melt adhesive composition useful for bonding plastics and metallized foil containers. The adhesive composition contains a blend of APAO, a solid benzoate plasticizer and a hydrocarbon tackifier.
Sustic, in U.S. Pat. No. 5,723,546, discloses a polymer blend consisting of a high molecular weight average, predominantly atactic flexible polyolefin polymer and a low molecular weight average APAO. The blend is said to be useful for hot melt adhesives.
Blending APAO with PE, PB or the conventional IPP leads to several drawbacks. The prior art adhesives containing APAO/PE or APAO/PB blends, such as, for example, those described herein above in U.S. Pat. Nos. 4,120,916, 4,761,450, and 5,478,891, tend to have poor compatibility. These adhesives can undergo phase separation during application process at which the hot melt adhesives have to be kept in the molten state at high temperature for a prolonged period of time, sometimes for hours or even days. Charring, skinning and gelling can develop rather quickly in the phase separated hot melt adhesives, thereby causing the application equipment to block or plug-up. The incompatibility of such polymer blends also imparts brittleness, optical haziness, poor or no open time, and low bond strength. Although APAO and the conventional IPP blend based hot melt do not have the compatibility problems, they still suffer from all the other drawbacks herein described above. Moreover, due to high crystallinity and high melting point of the conventional IPP polymers, hot melt adhesives based on APAO/IPP blends tend to be hard and brittle unless the IPP polymer amount is kept at a very low level, such as, for example, at about or below 2% by weight as disclosed in the prior art U.S. Pat. No. 4,022,728. As a result, these adhesives will have poor tensile strength, poor bond strength and poor impact resistance. Another detrimental effect of IPP is the increased coating temperature. The adhesive must be heated above the melting point of IPP (ranging from 180 to 200xc2x0 C.) for it to reach a liquid state. The blend of high and low molecular weight atactic polyolefin approach described in U.S. Pat. No. 5,723,546, although offering some improvement on tensile properties of APAO, has not been able to provide sufficient tensile strength and high temperature properties to overcome the deficiencies of sole APAO based hot melts.
In a prior U.S. Pat. No. 5,317,070, Brant et al disclosed a hot melt adhesive based on tackified SPP having a polymer chain of at least 80% racemic dyads and having a melting point of about 100 to 180xc2x0 C. The adhesive is claimed to have good open times between the application of the adhesive and the formation of the joint. This type of tackified SPP usually lacks flexibility and toughness, and therefore, will also have poor bond strength and poor impact resistance. Furthermore, SPP exhibits an inherent shrinkage problem when it transforms from liquid to solid crystalline state. The shrinkage often causes stress concentration at adhesive/substrate interfaces, and consequently, catastrophic bond failure.
It therefore would be advantageous to provide a hot melt adhesive that will overcome the shortcomings of the prior art adhesives herein mentioned above. It is found in the present invention that a synergistic effect exists in an SPP/APAO blend hot melt composition. Due to this synergy, an SPP/APAO polymer blend possess a unique combination of properties which the prior art APAO and SPP based hot melt systems have failed to offer.
The present invention is directed to a hot melt adhesive composition based on a polymer blend of the SPP and APAO. The adhesive comprises, in addition to the SPP/APAO blend, a tackifying resin, an optional plasticizer and an optional wax as the primary ingredients. The composition of the present invention takes advantage of complimentary properties between SPP and APAO and has overcome the shortcomings of the prior art APAO blend adhesives and tackified SPPs. The composition of the present invention provides well balanced properties of tensile strength, toughness, flexibility and adhesion. It shows complete compatibility, excellent heat stability, adjustable open time, improved cohesive strength, low viscosity, low shrinkage upon solidification, low or no tack when set, and good processibility with conventional coating equipment. In particular, the present invention leads to an adhesive composition that is well suited for a variety of spray coating application techniques, such as, for example, spiral spray, melt-blown, control coat, control wave and the like, whereas the prior art APAO and SPP based adhesives lack such broad processibility.
One of the prominent features of the hot melt adhesive composition of the present invention is its ability to provide a strong bond to a variety polar and nonpolar substrates at very low coating weight. The adhesive works well on both porous and film substrates. At the equal coating weight, the present adhesive will yield a much higher peel adhesion value than the prior art adhesives. The ability to yield high peel strength at low coating weight enables the end user to use less adhesive, which is obviously a great cost benefit.
One objective of the present invention is to provide an adhesive that has very low or no tack at ambient temperature. This feature is particularly advantageous for use on porous substrates where adhesive bleed-through and the subsequent blocking are of great concern. Blocking is especially disastrous in the manufacturing of roll-goods. The roll goods are usually intermittent products that will be converted to end products in a subsequent process. Roll blocking makes it difficult, and sometimes even impossible to unwind the roll in the subsequent converting process. The non-tacky characteristic of the present adhesive in combination with low coating weight capability will eliminate the blocking problem.
Another objective of the present invention is directed towards a sprayable hot melt adhesive for construction of disposable nonwoven articles for binding polyethylene, polypropylene films, nonwoven fabrics and the like to each other and to themselves. The adhesive provides excellent peel strength and bond durability in such application.
Another objective of the present invention is to provide a sprayable hot melt for elastic attachment application in manufacturing baby diapers, adult incontinent briefs and the like for binding elastic strands between a polyethylene film and a nonwoven fabric, or between two nonwoven fabrics. This type of adhesive can be formulated to have dual functions for both elastic attachment and construction.
Another objective of the present invention is to provide a hot melt adhesive for carton and case sealing to provide a strong bond at sub-ambient temperatures. Due to its toughness and flexibility, an SPP/APAO based hot melt is advantageous for low temperature applications. The adhesive of the present invention would offer fiber tear bond at the ambient temperature.
The hot melt adhesive composition of the present invention comprises as components thereof a mixture of the following ingredients:
a. A blend of syndiotactic polypropylene (SPP) polymer and atactic poly-xcex1-olefin (APAO) in the amount of about 15% to 80% by weight, preferably in the amount of about 25% to 60% by weight, and most preferably in an amount of from about 35% to 55% by weight, said blend having a SPP to APAO ratio from about 10% by weight of SPP and 90% by weight of APAO to about 90% by weight of SPP and 10% by weight of APAO; said blend having a most preferred ratio of 20% SPP and 80% APAO; said SPP having a density of about 0.85 g/cc to 0.92 g/cc and a melt flow rate of equal to or greater than 1 g/10 min and a r value equal to or greater than 70%; and said APAO having a density of about 0.85 g/cc to 0.89 g/cc and a glass transition temperature (Tg) of from about xe2x88x925 to xe2x88x9240xc2x0 C. and a weight average molecular weight (Mw) of from about 4,000 g/mol to about 150,000 g/mol.;
b. A compatible tackifier in the amount of 15% by weight to 65% by weight, preferably in the amount of 30% by weight to 50% by weight;
c. About 0% to 35% by weight, preferably about 5% to 30% by weight, of a plasticizer;
d. About 0% to 3% by weight of a stabilizer or antioxidant.
e. Optionally, about 0% to 30% by weight, preferably about 0% to 18% by weight, of a wax;
the components of the composition adding up to 100% by weight. The adhesive composition may contain other components such as a filler and/or a colorant and/or a fluorescing agent and/or another polymer that can modify the adhesive properties of the above basic adhesive composition, as desired.